Device for reconstructing a graphical message

ABSTRACT

A device comprising receiving means for receiving a sequence of information units, a first display arranged for displaying the sequence of information units by activating cells in a first electro-optical layer in dependence on the sequence, a second display arranged for activating cells in a second electro-optical layer in dependence on elements in a key sequence, in which the first and second displays are arranged to be superimposed on each other, and in which one of the first and second displays comprises a light valve, and the other of the first and second displays comprises one of: a transmissive display, an emissive display, a reflective display and a transflective display.

The invention relates to a device for reconstructing a graphicalmessage.

Visual cryptography (M. Naor, A. Shamir: Visual Cryptology, Eurocrypt'94, Springer-Verlag LNCS Vol. 950, Springer-Verlag, 1995, pp 1-12) canbriefly be described as follows. An image is split into two randomizedparts, the image plus a randomization and the randomization itself.Either part contains no information on the original image because of therandomization. However, when both parts are physically overlaid theoriginal image is reconstructed. An example is given in FIG. 1: originalimage 100 is split into shares 110 and 120, which when overlaid resultin reconstructed image 130.

If the two parts do not fit together, no information on the originalimage is revealed and a random image is produced. Therefore if twoparties want to communicate using visual cryptography, they have toshare the randomization. A basic implementation would be to give areceiving party a transparency containing the randomization. The senderwould then use this randomization to randomize the original message, andtransmits the randomized message to the receiver, on a transparency orby any other means. The receiver puts the two transparencies on top ofeach other and recovers the message. This, scheme can be compared to aone-time pad.

A more flexible implementation is obtained when using two displayscreens, e.g. two LCD screens. A first liquid crystal display rendersthe image plus randomization and a second LCD displays the randomizationitself. If the screens are put on top of each other, the reconstructedimage appears.

FIG. 2 illustrates the visual cryptography process as devised by Naorand Shamir in the above-referenced paper. The process is illustratedhere for a single pixel, but of course every pixel in the source imageis to be processed in this way.

Every pixel of the original image 100 is translated to four sub-pixels.To generate the first share S1 for this pixel, two of the four pixelsare randomly chosen to be black (non-transparent) while the other twoare chosen to be white (transparent). To generate the other share S2 ofthis pixel the four sub-pixels are copied if the corresponding pixel inthe original image was white and they are inverted if the original pixelwas black. For each pixel a new random choice of which two of the fourpixels should be black (non-transparent) needs to be made. The number ofsub-pixels into which the pixels are split can be chosen arbitrarily,but should be at least two.

This way, two collections of sub-pixels are formed. These collectionsmake up the two shares. Neither of the shares gives any information onthe color of the original pixel. In all cases, some of the sub-pixelschosen to represent the original pixel in either of the shares are blackand the rest is white. Further, all possible combinations of black andwhite are equally likely to occur, since the random choice is made witha probability of p=0.5, independently for each pixel.

To reconstruct the original image, the two shares S1 and S2 are to besuperimposed, i.e. put on top of each other. This is shown in the lastcolumn (R) of FIG. 2. If the original pixel were black (P2), then thesuperposition of the sub-pixels from shares S1 and S2 will result infour black sub-pixels. If the original pixel where white (P1), then thesuperposition of the sub-pixels from shares S1 and S2 will result in ablack and white pattern in the reconstructed image 130, which oftenappears to be gray when seen from a distance.

If the two parts do not fit together no information on the originalimage is revealed and a random image is produced. Without knowing bothof the shares, the probability that one set of sub-pixels corresponds toa white pixel in the original image 100 is equal to the probability thatthat set corresponds to a black pixel in the original image 100.

It is clear that the above scheme suffers from several disadvantages.First, in order to show the same level of detail in the reconstructedimage 130, the shares 110, 120 require a four times higher resolutionthan the original image 100. This makes the reconstructed image 130 fourtimes as large as the original image 100.

Further, the contrast and brightness of the reconstructed image 130 isseverely reduced compared to the contrast and brightness of the originalimage 100. This is due to the fact that white pixels in the originalimage 100 turn into a pattern of black and white pixels in thereconstructed image 130. This also causes a small distortion at theedges of the parts that were black in the original image 100. Theseeffects can be seen clearly in FIG. 1.

European patent application 02075178.0 (attorney docket PHNL020050) bythe same applicant as the present application provides a method anddevice for reconstructing a graphical message. After receiving thesequence of information units, preferably a sequence of binary values,the device renders the sequence on a first display, without performingany processing or decrypting steps before any displaying takes place.The information units are displayed as they are received. On a seconddisplay another pattern is displayed, which is generated based entirelyon a key sequence. This European patent application is incorporated byreference in the present application.

Reconstruction of the image is performed by superimposing the first andsecond displays in the correct alignment, so that the user can see thereconstructed graphical message. The reconstruction is performeddirectly by the human eye and not by a device which might becompromised. This makes the use of visual cryptography to communicatesecret information more secure.

The prior art visual cryptography systems, as explained above, rely uponthe use of polarized light to maintain the sharpness and clarity of theoriginal image in the reconstruction. However, the use of polarizedlight seriously decreases the brightness of a display, by about 50%.Furthermore, the above mentioned prior art visual cryptography systemrequires that (at least a portion of) the panels are in direct contactwithout an intermediate polarizer being present. As a consequence, suchpanels or screens (for reasons of convenience hereafter called“displays”) must be customized for cryptography use.

It is an object of the present invention to provide a device for usewith visual cryptographic applications which is more effective andprovides a display with on the average more brightness than theabove-mentioned displays when used with visual cryptographicapplications.

This object is achieved according to the invention in a device asclaimed in claim 1. These displays are easy and cheap to implement, donot require customization, do not rely upon polarized light and due tothe combination of light valve and transmissive, emissive, reflective ortransflective display produce on the average a brighter reconstructedimage than the prior art.

If one of the displays is embodied as a reflective display, it ispreferably realized as a combination of a light source and one of: aliquid crystal display, an electrochromic display, an electromechanicaldisplay, an electrowetting display and an electrophoretic display, or asa hybrid mirror, as described in international patent applicationPCT/IB01/02516 (attorney docket PHNL010007) by the same applicant as thepresent application.

A color filter or other color rendering method (such as providing thedisplay pixels themselves with intrinsic color) can be provided in thedisplay to have the reconstructed image appear in any desired color.Various other advantageous embodiments are set out in the dependentclaims.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments shown in the drawings, inwhich:

FIG. 1 shows an original image, two shares obtained by visuallyencrypting the original image and a reconstructed image obtained bysuperimposing the two shares;

FIG. 2 illustrates the visual cryptography process as devised by Naorand Shamir in the above-referenced paper;

FIGS. 3A, 3B, 4, 5A and 5B illustrate various embodiments of a devicecapable of reconstructing a visually encrypted message;

FIG. 6 illustrates a variant on any of these embodiments in which acolor filter is provided; and

FIG. 7 illustrates how the two displays in the device can additionallybe used to present two separate images to a user.

Throughout the figures, the same reference numerals indicate similar orcorresponding features. Some of the features indicated in the drawingsare typically implemented in software, and as such represent softwareentities, such as software modules or objects.

The above-mentioned European patent application 02075178.0 (attorneydocket PHNL020050) extensively describes the basic construction of adevice comprising receiving means for receiving a sequence ofinformation units, a first display arranged for displaying the sequenceof information units by activating cells in a first electro-opticallayer in dependence on the sequence, a second display arranged foractivating cells in a second electro-optical layer in dependence onelements in a key sequence, in which the first and second displays arearranged to be superimposed on each other. For reasons of brevity, thisdescription is not repeated here. The present invention provides variousadvantageous embodiments of the first and second displays of thatdevice.

In general, the device according to the invention comprises two matrixdisplays with respective electro-optical layers. The first layerdisplays the sequence as it is received from a server. The second layerdisplays a pattern based on a key sequence, for example by generating apseudo-random bit sequence using the key sequence as a seed toinitialize a pseudo-random number generator (PRNG). Cells in the seconddisplay are then activated (e.g. a sufficient voltage is applied tothem) in dependence on the pseudo-random bit sequence.

When the first and second displays are subsequently superimposed on eachother, the message is visually reconstructed. Because both displays eacheffectively display one share of a visually encrypted image, the usercan now observe the reconstructed image representing the message.

Preferably the second display is embodied in a unit physically separablefrom the first display, and provided with a memory for storing the keysequence. If the key sequence is used in conjunction with a PRNG, or asinput for a symmetric cipher or other technique to generate a bitsequence from which the pattern can be reconstructed, then also aprocessor is necessary in the unit. The unit can then be placed on topof the display in a host device, or inserted in a slot in the hostdevice to superimpose the two displays. The separable unit can inprinciple be considerably smaller than the first display, significantlyreducing the cost of constructing this unit.

No electrical, optical or other communication paths between the firstand second displays, or the unit and host device in which they areembodied, should exist. As the patterns and the key sequence areprovided in digital (electronic) form, any such communication pathscould potentially be abused by an attacker to obtain patterns and/or keysequence.

In FIG. 3A, the lower layer LL comprises an emissive display, e.g.Polymer-LED, an OLED, a field emission display or a CRT. The upper layerUL comprises a light valve with transparent material which can be eithermade absorbing, reflecting or scattering. Light valves are matrixdisplays which spatially modulate the transmission of incident light.Some examples are LCDs, electrophoretic displays, electrochromicdisplays, electrowetting displays and hybrid mirrors when operating in amode between transparent and either absorbing, reflecting or scatteringoptical states.

By superimposing the upper layer UL (the separable unit) on the lowerlayer LL (the host device) the two complementary patterns produced bythe respective displays are combined and the original image IMG isreconstructed. FIG. 3B is a variant of the embodiment of FIG. 3A, inwhich the lower layer LL is a light valve with an external light source.

In FIG. 4 another embodiment is shown, wherein the upper layer ULcomprises a light valve display, and the lower layer comprises anintrinsically reflective display such as electrophoretic displays, PDLCor guest-host materials. Other display types may also comprise areflective component to facilitate the reflection. Examples are LCDs,electrochromic displays, electrowetting displays and hybrid mirrors,which could be used when operating in a mode between reflective orscattering and absorbing or transmitting optical states. The lower layeris now used to reflect the incoming light at certain pixels and totransmit or absorb it at others.

In this embodiment, the lower layer LL is provided in the separate unitmentioned above on which the pattern represented by the key sequence isdisplayed. In case the lower layer LL is bi-stable, then it can bedriven at a very low power and is therefore very well suited for smalldevices. This way the separate unit only has very modest powerrequirements. Preferably the host device is provided with a slot inwhich the unit can be inserted. This makes properly positioning theupper and lower layers very easy for a user.

FIGS. 5A-B show yet another embodiment, in which the upper layer UL is alight valve display, and the lower layer LL is a transflective display.Again, the lower layer LL is provided in the unit, and the upper layerUL is provided in the host device.

This embodiment can be used in both a reflective or a transmissive mode.The lower layer LL is made of transparent material which can be eithermade absorbing, reflecting or scattering. Examples are electrophoreticdisplays, PDLC, guest-host materials. In reflective mode, the lowerlayer is used to reflect the incoming light at certain parts (typicallypixels) and to absorb or scatter it at others. In the transmissive modethe lower layer is used to transmit the light from a backlight BL atcertain pixels and to absorb, scatter or reflect it at others.

In FIG. 5A the backlight BL has been activated, and the lower layer LLis now used in its transmissive mode. In FIG. 5B the backlight BL is notactivated, which means that the lower layer LL is used in the reflectivemode.

Any of the above embodiments can be enhanced with a color filter, tohave the resulting image IMG appear in any desired color. FIG. 6illustrates a variation of the embodiment of FIG. 3A in which a colorfilter FIL has been installed directly above the upper layer UL. It willbe evident that this variation can equally well be applied to any of theother embodiments. The color filter FIL could also be provided betweenthe upper layer UL and the lower layer LL. Alternatively, both displayscan be provided with color filters.

It is worth noting that a combination of for example an upper layer ULswitching between scattering and transmissive mode, and a lower layer LLswitching between reflective and absorbing mode provides an additionalbenefit, as illustrated in FIG. 7. The two displays can nowindependently be used to generate images which can be viewed from twosides. One image IMG1 appears on one side of the device, but at the sametime another image IMG2 appears on another side of the device.

An important application for such displays are telephones where thedisplay flips open. Either of the layers UL and LL can be positioned tobe at the outside when the display is closed. This way, a message suchas “There is a visually encrypted message waiting for you” could beshown on this “outside” layer as image IMG1 or IMG2 so that the user cansee it. He can then choose to flip open the display and switch thedevice to cryptographic mode. In this mode, the lower layer LL displaysthe patterns depending on the key sequence, and the upper layer ULdisplays the patterns depending on the received sequence (or viceversa). The user can then see the reconstructed image IMG, in a mannersimilar to that shown in FIG. 4.

The invention can be used to transmit a wide variety of messages fromserver to the device. For example, sensitive information like a bankbalance, a private e-mail message, a new PIN code or password can beprovided securely to the operator of the device.

One particularly useful application is to securely allow composition ofa message by the operator of the device. In this embodiment, thereconstructed message represents a plurality of input means such as keyson a keyboard. Each input means represents an input word that can beused in the message that will be composed by the user. Such keys couldbe visually rendered as keys representing different alphanumericalcharacters, or as buttons representing choices like ‘Yes’, ‘No’, ‘Moreinformation’ and so on. Next to keys, the input means could also becheckboxes, selection lists, sliders or other elements typically used inuser interfaces to facilitate user input.

The user now applies pressure to a particular spot of a touch-sensitivelayer provided near or in the upper layer UL to select particular inputmeans. The device then derives a set of coordinates at which pressurewas applied, and sends this set of coordinates to the server. The devicecannot learn which input means were selected, as this can only be foundout when the reconstructed image is available. And because visualcryptography is used, the reconstructed image is only visible to theuser, not stored anywhere in the device.

The invention can be used in any kind of device in which a securecommunication from a server to a client device and/or vice versa isnecessary. The device can be embodied as a personal computer, laptop,mobile phone, palmtop computer, automated teller machine, publicInternet access terminal, or in fact any client device that is notcompletely trusted by its user to not contain any malicious software orhardware.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word “comprising” does notexclude the presence of elements or steps other than those listed in aclaim. The word “a” or “an” preceding an element does not exclude thepresence of a plurality of such elements.

The invention can be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. Inthe device claim enumerating several means, several of these means canbe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

1. A device comprising receiving means for receiving a sequence ofinformation units, a first display arranged for displaying the sequenceof information units by activating cells in a first electro-opticallayer in dependence on the sequence, a second display arranged foractivating cells in a second electro-optical layer in dependence onelements in a key sequence, in which the first and second displays arearranged to be superimposed on each other, and in which one of the firstand second displays comprises a light valve, and the other of the firstand second displays comprises one of: a transmissive display, anemissive display, a reflective display and a transflective display. 2.The device of claim 1, in which the other display comprises acombination of a light source and one of: a liquid crystal display, anelectrochromic display, an electromechanical display, an electrowettingdisplay, an electrophoretic display and a hybrid mirror.
 3. The deviceof claim 1, further comprising a color filter superimposed on at leastone of the first and second displays.
 4. The device of claim 1, furthercomprising a touch-sensitive layer, using which input representing a setof coordinates can be received from a user, and transmitting means fortransmitting the received input to a server.
 5. The device of claim 1,in which the second display is embodied in a unit physically separablefrom the first display, and provided with a memory for storing the keysequence.
 6. The device of claim 5, in which the unit further comprisesa processor for generating a pseudo-random stream of bits in dependenceon a portion of the key sequence, the second display being arranged foractivating the cells in the second electro-optical layer in dependenceon the stream of bits.
 7. The device of claim 5, in which the devicefurther comprises a slot in which the unit can be inserted.